Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian brain. GABA exerts its physiological actions in the brain via the activation of two distinct types of receptor: GABA-A receptors, which are ligand-gated ion channels, and GABA-B receptors, which are G proteincoupled receptors. GABA-A and GABA-B receptors are known to exhibit forms of cross-talk and mutual regulation for which no mechanism has been defined. This project aims to study the importance of a novel and direct interaction found between the GABA-BR1 receptor and the gamma2 subunit of the GABA-A receptor. This physical association may provide a mechanism to allow for direct cross-talk between GABA-A and GABA-B receptors. The structural determinants and physiological significance of this interaction, however, are completely unknown at the present time. The specific regions of GABA-BR1 and the gamma2 subunit of the GABA-A receptor involved in mediating their interaction will be elucidated using a mutagenesis approach in combination with both co-immunoprecipitation and fusion protein pull-down studies. The effects of GABA-A receptor association on GABA-B receptor pharmacology will be studied in ligand binding assays, and GABA-A receptor modulation of GABA-B receptor signaling and internalization will also be analyzed. Furthermore, GABA-B receptor regulation of GABA-A receptor pharmacology, channel activity and phosphorylation will be examined, with an emphasis on determining the functional importance of the direct interaction between GABA-BR1 and the GABA-A receptor gamma2 subunit. These studies will shed new light on the regulation of cellular responses to GABA and the molecular basis for cross-talk between GABA-A and GABA-B receptors. Such information is critical for a comprehensive understanding of pharmaceuticals acting on GABA receptors. GABA-A receptors are the targets for such commonly prescribed therapeutic drugs as benzodiazepines and barbiturates, while the more recently-identified GABA-B receptors represent excellent potential targets for novel therapeutic drugs aimed at treating disorders such as schizophrenia, epilepsy, anxiety, chronic pain and depression.